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Non-native pests, climate change, and their interactions are likely to alter relationships between trees and tree-associated organisms with consequences for forest health. To understand and predict such changes, factors structuring tree-associated communities need to be determined. Here, we analysed the data consisting of records of insects and fungi collected from dormant twigs from 155 tree species at 51 botanical gardens or arboreta in 32 countries. Generalized dissimilarity models revealed similar relative importance of studied climatic, host-related and geographic factors on differences in tree-associated communities. Mean annual temperature, phylogenetic distance between hosts and geographic distance between locations were the major drivers of dissimilarities. The increasing importance of high temperatures on differences in studied communities indicate that climate change could affect tree-associated organisms directly and indirectly through host range shifts. Insect and fungal communities were more similar between closely related vs. distant hosts suggesting that host range shifts may facilitate the emergence of new pests. Moreover, dissimilarities among tree-associated communities increased with geographic distance suggesting that human-mediated transport may facilitate the introductions of new pests. The results of this study highlight the need to limit the establishment of tree pests and increase the resilience of forest ecosystems to changes in climate.

Live plants are often traded in winter or early spring when the number of pests is potentially low, in part because deciduous trees lack foliage. However, the pests that may be associated with traded dormant plant material are likely to be largely unknown. Pest risk assessments can only be performed for known pest species and it is therefore important to identify potential pests before they are moved to a new area. In addition, it is difficult to predict whether introduced pests, even if already well known, may shift host after their arrival. Several studies attempted to estimate the likelihood of host shifts using literature about known pest-host associations, but the number of pests in these studies was often limited and the data were unlikely collected in the same place and at the same time. To overcome these problems, we performed a global study which aimed at detecting potential insect pests and fungal pathogens on congeneric native and exotic woody plant species that belong to six studied tree genera. Sampling was done simultaneously in 33 countries around the world following a common sampling protocol. At each location, twenty 50 cm long, dormant twigs were collected from each tree species and kept in containers with water at room temperature. Emerged insects were collected and identified based on morphology and DNA barcoding of the mDNA COI region. Fungal DNA was extracted from pooled buds, twig parts and needles for species identification using a metabarcoding approach based on the rDNA ITS region. Here, we present preliminary results of this study and discuss them in the context of pest risk assessment.